<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <style>
      .board {
        width: 200px;
        height: 200px;
        border: 3px solid #654321;
        border-radius: 3px;
        margin: 5px;
        background-color: chocolate;
        background-image: linear-gradient(
            to bottom,
            transparent 19px,
            #654321 19px
          ),
          linear-gradient(to right, transparent 19px, #654321 19px);
        background-size: 20px 20px;
        /* background-clip: content-box, border-box; */
      }

      .item {
        display: flex;
        float: left;
        justify-content: center;
        width: 20px;
        height: 20px;
      }
    </style>

    <div class="board">
      <div class="item">1</div>
      <div class="item">2</div>
      <div class="item">3</div>
      <div class="item">4</div>
      <div class="item">5</div>
      <div class="item">6</div>
      <div class="item">7</div>
      <div class="item">8</div>
      <div class="item">9</div>
      <div class="item">10</div>
      <div class="item">11</div>
      <div class="item">12</div>
      <div class="item">13</div>
    </div>
  </body>
</html>
